THE COMPLETE MOLECULAR GEOMETRY OF SALICYL ALDEHYDE FROM ROTATIONAL SPECTROSCOPY

Abstract

Salicyl aldehyde is a well known planar molecule containing an internal hydrogen bond. In preparing the publication of our previous report of the study of its rotational spectrum {Z. Kisiel et. al., 61st OSU Symposium on Molecular Spectroscopy, The Ohio State University, Ohio 2006, RI-12} we have taken the opportunity to update the structure determination of this molecule to the complete $r_e^{\rm SE}$ geometry. The molecule contains 15 atoms and we have used supersonic expansion FTMW spectroscopy to obtain rotational constants for a total 26 different isotopic species, including all singly substitued species relative to the parent molecule. The $^{13}$C and $^{18}$O substitutions were measured in natural abundance, while deuterium substitutions were carried out synthetically. The $r_e^{\rm SE}$ determination requires the calculation of vibration-rotation changes in rotational constants from an $ab~initio$ anharmonic force field, which necessitates some compromises in the level of calculation for a molecule of the size of salicyl aldehyde. For this reason we studied the five lowest vibrationally excited states, by using the combination of room-temperature mm-wave spectroscopy and waveguide Fourier transform cm-wave spectroscopy. The experimental excited state rotational constants were then used to calibrate the anharmonic force field calculation. The resulting $r_e^{\rm SE}$ geometry is compared with other types of geometry determination possible from this data, with emphasis on the effect of the near zero principal coordinate of the important C$_2$ atom.

Author Institution: Institute of Physics, Polish Academy of Sciences, Al. Lotnikow; 32/46, 02-668 Warszawa, Poland; Department of Chemistry, University of Warsaw; Pasteura 1, 02-093 Warszawa, Poland; Institut fur Physikalische Chemie, Christian-Albrechts-Universitat zu Kiel; Olshausenstrasse 40, D-24098 Kiel, Germany